A typical DC-to-AC inverter energized from a 12-volt DC input signal uses a single stage inverter circuit to produce a quasi-sine wave output signal. The peak output signal amplitude for conventional converters of this type often varies over a wide range, e.g. from 110 volts peak to over 200 volts peak for a nominal specified output of 120 volts rms. Third and other odd order harmonics in the output of these devices are usually quite high, and because the duty cycle of the output signal typically varies with load, the harmonic content is difficult to control. Variations in the power required by the load is directly reflected back to the DC source since the conventional DC-to-AC inverter does not include adequate provision for storing energy to meet even short term peak current demands. As a result, the ratio of peak to average current demand on the source can be quite high, causing the overall efficiency of the conventional inverter to be relatively low.
Ideally, the input current to a inverter should remain constant during short term variations in the load. Most conventional inverters draw current from the DC source in a quasi-sine wave pattern that is similar to their output voltage waveform. The average current from the source for such devices is significantly higher than if the current were supplied at a substantially constant level. Due to their inefficiencies, the typical inverter tends to be relatively heavy, requiring a larger transformer than would be necessary for providing a given load current with a generally constant supply current.
In consideration of these problems with existing DC-to-AC inverters, it is an object of the present invention to provide a DC-to-AC power converter that regulates the voltage applied to a varying load without varying the pulse width of the output signal. A further object is to provide such a converter with a fixed duty cycle selected so as to minimize third harmonic content in the signal supplied to the load. Another object is to provide a two stage DC-to-AC converter that draws a generally constant current from a source of DC, meeting peak load demands with energy stored in the converter. Yet another object is to provide separate voltage and current feedback loops to control and regulate the conversion process. These and other objects and advantages of the present invention will be apparent from the attached drawings and the description of the preferred embodiment that follows.